Consolidation of long-term synaptic plasticity - an important cellular mechanism underlying long-term memory - requires new macromolecular synthesis (genes and proteins). In many cases additional training or stimuli enhance the duration and amplitude of the long-term memory/synaptic plasticity by activating many of the same neurons and synapses activated during the initial training. However, exposure to the additional stimuli typically introduces a labile feature in the pre-existing memory/plasticity such that timely disruptions in critical cellular processes often produce amnesia-like responses for both the memory and the underlying cellular plasticity. This important neurobiological phenomenon is the basis for developing therapies to alleviate post- traumatic stress disorders. It remains unclear however: 1) how additional training or stimuli might prevent the natural reversal of macromolecular synthesis-dependent long-term memory/cellular plasticity evoked by the initial training or stimuli, 2) how additional training or stimuli evoke the labile property of the pre-existing long- term memory/cellular plasticity, and 3) whether additional stimuli that produce more persistent forms of long- term memory/cellular plasticity activate the same molecules and pathways required to initiate the long-term memory/plasticity. This proposal will address these questions using a tractable model system consisting of a behaviorally relevant synapse reconstituted in cell culture. The sensory-motor neuron synapse of Aplysia expresses different forms of macromolecular synthesis-dependent long-term facilitation (non-associative or associative LTF - cellular correlates of sensitization and classical conditioning of defensive reflexes) of variable duration that depends on the number of stimuli. Disrupting specific cellular processes during the additional stimuli produces an amnesia-like response, and the additional stimuli activate new signaling pathways to regulate the persistent synthesis, secretion and downstream signaling of a neurotrophin-like neuropeptide sensorin required for persistent long-term plasticity. Natural reduction in sensorin synthesis or downstream signaling following the initial stimuli or precipitous reduction in sensorin synthesis or downstream signaling by various manipulations during the additional stimuli leads to a reversal of the plasticity. With this tractable system consisting of large identified neurons and their synaptic connection that is amenable to cell-specific manipulations of specific molecules by intracellular injections, and allowing direct assays of gene or protein expression and synapse structure and strength, the following hypotheses will be explored: 1) The reversal of long-term plasticity (natural or when specific inhibitors are added during the additional stimuli) is mediated by specific homeostatic mechanisms affecting either the number of presynaptic varicosities (new or pre-existing) and/or postsynaptic receptor sensitivity/distribution at presynaptic varicosities. 2) The labile nature of the pre-existing long-term plasticity during the additional stimulation is produced by a disruption at various stages in a positive auto-regulatory feedback of downstream signaling of the secreted neurotrophin-like peptide sensorin and the transcription/translation of sensorin mRNA. 3) Different forms of persistent LTF converge by regulating this auto-regulatory feedback loop, but are mediated by different cell-specific mechanisms and signaling pathways.